X-ray phase contrast imaging is an x-ray imaging method that, unlike conventional x-ray instruments, does not only use the absorption by an object as source of information. X-ray phase contrast imaging combines the absorption with the shift of the phase of the x-ray radiation when passing through the object. The information content is disproportionately higher because the absorption supplies accurate images of the strongly absorbing bones and the phase contrast makes sharp images of the structures of the soft tissue. This offers the possibility of being able to identify pathological changes, such as the emergence of tumors, vessel constrictions, or pathological changes of cartilage, substantially earlier than previously.
The passage of x-ray radiation through matter is described by a complex refractive index. The imaginary part of the refractive index specifies the strength of the absorption, whereas the real part of the refractive index specifies the phase shift of the x-ray wave passing through a material. During phase contrast imaging, the phase information of the local phase or of the local gradient of the phase of the wavefront passing through an object is determined. Analogously to x-ray tomography, it is also possible to reconstruct tomographic representations of the phase shift based on a multiplicity of images.
There are a number of options for realizing x-ray phase contrast imaging. Known solutions are directed to making the phase shift of the x-ray radiation visible as an intensity variation when passing through an object via special arrangements and methods. One method is grating phase contrast imaging, also referred to as Talbot-Lau interferometry (e.g., European Patent Application No. EP 1 879 020 A1). The essential part of the Talbot-Lau interferometer consists of three x-ray gratings arranged between an x-ray tube and an x-ray detector.
In addition to the conventional absorption image, such interferometers may depict two additional measurement parameters in the form of further images: the phase contrast image and the dark-field image. The phase of the x-ray wave is determined by interference with a reference wave by using the interferometric grating arrangement.
For example, European Patent Application No. EP 1 879 020 A1 discloses an arrangement including an x-ray tube and a pixelated x-ray detector, between which an object to be irradiated is arranged. A source grating, also referred to as a coherence grating, is arranged between the focal point of the x-ray tube and the object. The source grating serves to simulate a plurality of line sources with a spatial partial coherence of the x-ray radiation, a precondition for interferometric imaging.
A diffraction grating, also referred to as a phase grating or Talbot grating, is arranged between the object and the x-ray detector. The diffraction grating impresses a phase shift (e.g., typically pi) onto the phase of the wavefront.
An absorption grating between the diffraction grating and the x-ray detector serves for measuring the phase shift generated by the object. The wavefront upstream of the object is “bent” by the object. The three gratings are to be arranged parallel to one another and at exact distances from one another.
The x-ray detector serves for the spatially dependent detection of x-ray quanta. Because the pixelation of the x-ray detector is generally not sufficient to resolve the interference strips of the Talbot pattern, the intensity pattern is scanned by shifting one of the gratings (e.g., “phase stepping”). Scanning is performed step-by-step or continuously in a direction perpendicular to a direction of the x-ray beam and perpendicular to the slit direction of the absorption grating. Three different types of x-ray images, the absorption image, the phase contrast image, and the dark-field image, may be recorded or reconstructed.
As an alternative to “phase stepping,” the “slot scanning method,” where the table with the object is displaced relative to the x-ray emitter and the gratings, may be used. By way of example, PCT Application No. WO 2013/111050 A1 describes such a method. The gratings may also be selected to be smaller, or partial gratings arranged next to one another may be used. In order to put together the individual images required for the slot-scanning method, the position of the object is to be known exactly in relation to the beam fan of the x-ray emitter. The main components of the system are mechanically connected to one another and equipped with complicated measurement aids. With known arrangements, a measurement accuracy of 0.3 to 1.0 mm is achievable.
In the case of a stationary patient table and a stationary detector in the slot scanning method, a fan beam of the x-ray emitter may be moved relative to the patient table and detector. However, for phase contrast imaging, the three x-ray gratings are also to be displaced in the process. Therefore, this option is more complicated.